As a panel structure including a plurality of panels, for example, a panel structure disclosed in Patent Literature 1 is described below. The panel structure includes a plate-like flat panel and a reinforcement panel in which a planar portion and a plurality of regular triangular pyramidal protrusions protruding from the planar portion are formed. The flat panel is joined to the summits of the plurality of regular triangular pyramidal protrusion portions of the reinforcement panel.